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<blockquote>quote:</font><hr>Originally posted by Dominic
The quote "A cool engine draws heat out of the combustion process - less vapor (combustion) pressure, less ability to do work (push the piston down)." is also extreme . The actual FLAME TEMPERATURE is the same everyime the cyl ignites, and by having cooler heads and block you will have a slightly tighter cyl area and desner incoming air - which in turn creates higher cyl pressures (slightly) once fired. The idea is not to have the engine as hot as possible, but to have the largest delta temperature (change in temps) between the power stoke and intake stroke. <hr></blockquote>
As you pointed out earlier, there is little expansion in iron, so the "slightly tighter cyl area" should be insignificant as well.
The way you make power, is to convert as much energy from the combustion of the fuel to mechanical work as possible. Thermal efficiency is a measure of the actual work per cycle vs the energy released during combustion. The main loss of energy is through heat. Any heat in the exhaust after it exits the combustion chamber is wasted, unless you use a turbo to extract it. Any heat lost to the cylinder walls is wasted as well. That is why high cylinder wall temperatures can help improve thermal efficiency. The temperature difference between cylinder air and the walls determines the rate of heat loss to the cylinder walls through convection. By minimizing the difference, you slow heat transfer, and retaining the energy within the cylinder. By keeping the energy within the cylinder, more energy can be converted to mechanical energy (work done on the piston by the air), and the thermal efficiency improves.
And for what ever its worth, Nox formation is greatest near stoiciometric conditions and high combustion temps, not cooler. Also, the flame temperature is not the same as the cylinder temp. When you are looking at peak cylinder temperature and pressure, I don't believe it is the same as the flame front temp.
One thing to point out in this debate is that it is nearly impossible to make a "singular" statement on how changing one engine parameter will affect performance. The difficulty with engines is that one change will effect everything else, which inturn causes more changes. For example, the cooler cylinder wall temps will effect numerous things. The intake change will start cooler as it is compressed, this will change the combustion temperetures which will alter the burn characteristics. These will affect the completeness of the reaction, and the heat losses, etc, etc, etc. Very interesting discussion.
[ June 14, 2002: Message edited by: Backfire ]</p>
The quote "A cool engine draws heat out of the combustion process - less vapor (combustion) pressure, less ability to do work (push the piston down)." is also extreme . The actual FLAME TEMPERATURE is the same everyime the cyl ignites, and by having cooler heads and block you will have a slightly tighter cyl area and desner incoming air - which in turn creates higher cyl pressures (slightly) once fired. The idea is not to have the engine as hot as possible, but to have the largest delta temperature (change in temps) between the power stoke and intake stroke. <hr></blockquote>
As you pointed out earlier, there is little expansion in iron, so the "slightly tighter cyl area" should be insignificant as well.
The way you make power, is to convert as much energy from the combustion of the fuel to mechanical work as possible. Thermal efficiency is a measure of the actual work per cycle vs the energy released during combustion. The main loss of energy is through heat. Any heat in the exhaust after it exits the combustion chamber is wasted, unless you use a turbo to extract it. Any heat lost to the cylinder walls is wasted as well. That is why high cylinder wall temperatures can help improve thermal efficiency. The temperature difference between cylinder air and the walls determines the rate of heat loss to the cylinder walls through convection. By minimizing the difference, you slow heat transfer, and retaining the energy within the cylinder. By keeping the energy within the cylinder, more energy can be converted to mechanical energy (work done on the piston by the air), and the thermal efficiency improves.
And for what ever its worth, Nox formation is greatest near stoiciometric conditions and high combustion temps, not cooler. Also, the flame temperature is not the same as the cylinder temp. When you are looking at peak cylinder temperature and pressure, I don't believe it is the same as the flame front temp.
One thing to point out in this debate is that it is nearly impossible to make a "singular" statement on how changing one engine parameter will affect performance. The difficulty with engines is that one change will effect everything else, which inturn causes more changes. For example, the cooler cylinder wall temps will effect numerous things. The intake change will start cooler as it is compressed, this will change the combustion temperetures which will alter the burn characteristics. These will affect the completeness of the reaction, and the heat losses, etc, etc, etc. Very interesting discussion.
[ June 14, 2002: Message edited by: Backfire ]</p>
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